LM324 quad opamp trouble

[Mizerable]

I made a headphone amp with the LM324. It's basically a Cmoy with the two left over opamps acting as voltage buffers.

The thing works except in some high frequency places ( ESPECIALLY piano sounds) it has this noise. The noise gets louder as I increase the volume at the source. However, it is not a problem with the source or headphones because there is no such noise ( or maybe its distortion ) with my other headphone amp.

I suspect it is the lm324. It does have a reputation for being a crappy audio op amp.

 

[tangent]

Yes, it's a crappy chip for audio, especially for direct-drive of headphones. But, that's not the only problem here.

The first is that this is a bipolar-input op-amp, not suited for the unmodified CMoy design. You will get high DC offset at the output if you haven't addressed this.

The second is that if the second set of op-amp channels is just in series with the outputs of the first set, they're not providing any benefit. You could parallel the two sets, a la A47, but fixing that still leaves you with the two problems above.

 

[Mizerable]

Parallel the two op amps ? I'm pretty new to this but does that mean:

Input from mp3 player into 3 gain opamp and 1 gain opamp at the same time then connect the two outputs together at the output jack?

 

[peranders]

The LM324 has a very special output stage. It runs in class C and the distortion is huge (as you may have noticed). This opamp is suitalble for other things but not audio.

 

[tangent]

Quote:

Originally Posted by Mizerable /img/forum/go_quote.gif Parallel the two op amps ? I'm pretty new to this but does that mean:

Why guess what it means, when a search here for "A47" will give you the actual answer?

 

[Mizerable]

Quote:

Originally Posted by tangent /img/forum/go_quote.gif Why guess what it means, when a search here for "A47" will give you the actual answer?

Yea did that. It means the output from the first amp goes into the second amp as well as the headphones

 

[tangent]

No, actually, it doesn't. Unlike in the CMoy, the output resistors in the A47 are required. If you just "connect the two outputs together at the output jack" without those resistors, you'll likely force one of the two op-amps in each channel into current-limiting mode, if it has it, or destruction if not.